The present invention relates to a turbine wheel containment deflector for attachment to the exhaust outlet of a fluid powered turbine engine starter motor.
Conventional turbine engines employ fluid or pneumatically powered starter motors coupled through a gear train to the turbine shaft to initially rotate the shaft of the turbine engine up to idle speed. The starter motors have a pneumatic turbine wheel housed within a turbine scroll. An exhaust duct is coupled to the scroll to direct the fluid axially rearwardly after it travels through the turbine wheel.
Occasionally pneumatic starters of this type fail as a consequence of prolonged free running of the starter turbine wheel at high rpm, on the order of 90,000 rpm. The free running condition is encountered when the starter air supply valve fails in the open position. Even though an external source of air may be cut off, most turbine engines are constructed so that bleed air from the compressor of the affected turbine engine is fed to the starter air duct. However, a properly operating air supply valve prevents the starter motor from being energized. This ducting arrangement permits cranking an engine with a previously started engine in the event an external source of air is not available. When the starter motor is exposed to the compressor bleed air, the starter will continue in a free run condition. The prolonged free run condition causes the bearings in which the turbine wheel shaft is journaled to fail. As this occurs, the turbine wheel shaft will disengage from the motor geartrain allowing the turbine wheel to move axially toward the exhaust duct within the scroll and allowing the blades of the turbine wheel to contact the interior surfaces of the scroll. The high rotational speed of the turbine wheel will cause the impeller blades to shear and melt as they contact the scroll, ultimately reducing the turbine wheel diameter sufficiently to allow it to exit through the exhaust duct. Moreover, as the turbine wheel enters the exhausted duct, fluid pressure builds up in the scroll behind the turbine wheel adding additional impetus to its axial expulsion.
When the turbine wheel is expelled through the exhaust duct, it is still rotating at a relatively high velocity. Since a turbine wheel of this type normally weighs on the order of four to five pounds, it can cause substantial damage to the surrounding turbine engine structure, nacelle structure, and adjacent equipment and personnel located on the ground. It is therefore a broad object of the present invention to provide a means by which the turbine wheel can be contained within or adjacent the pneumatic starter motor upon a failure of the type described above. An additional object of the present invention is to provide a containment means for the turbine wheel that can be attached to a pneumatic starter motor exhaust duct without modification to the duct. Further objects of the present invention are to provide a containment means that will dissipate the rotational energy of the starter turbine wheel upon failure, to provide a containment means that will not affect the normal performance characteristics of the starter motor, to provide a containment means that will relieve any air pressure increase behind a failed turbine wheel to prevent pressure buildup in the fan scroll and possible explosion of the scroll and motor housing, and to provide a simple, lightweight, turbine wheel containment means that can be installed on existing starter installations without modification or change to the starter motor.